Left 4 Dead 2

Left 4 Dead 2 is the direct sequal to the Co-op Game, left 4 dead. These were the weapons seen (albeit partially unfinished) in the E3 Gameplay Showcase.

Pistols

SIG P226

A SIG P226 is now the standard handgun for the Survivors. The pistol is completely stainless with could mean that it was based off the SIG Sauer P226 Elite Stainless, although that model features a beaver tail grip while the pistol in the game does not. It's equipped with an attached flash light/laser looking type mount, but as far is known no laser aiming system is used. Magazine capacity of 15 rounds.

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Glock

A Glock model is seen used as the second handgun for dual wielding, judging by the size of the magazine when counted on, it's a Glock 19. It does not feature the flashlight at the moment. Magazine capacity is also 15 rounds.

Shotguns

Remington 870

A Remington 870 Marine Magnum is seen with a Chrome finish. It is speculated that it's a direct clone of the previous shotgun.

Benelli M1014

The combat shotgun from the first game seems to be reintroduced in Left 4 Dead Two. It can be seen in the latest demo video showing the swamp campaign.

SPAS 12

The SPAS 12 is featured as the resident Auto Shotgun and used by most of the characters in the E3 Gameplay footage. It carries 10 rounds and uses the same animations from the previous auto shotgun (likely placeholders). Unlike many games, it is fired semi-auto.

Recent Gameplay footage has shown that it can also fire incendiary rounds.

SMGs

MAC-10

Seen fitted with the legendary Mitchell Werbell Mac-10 surpressor. Will likely replace the Uzi from the last game. It appears to have a flashlight attached to the surpressor with a hose clamp. In Demo footage it seems to be able to fire Incendiary type ammunition. It has a unreal 50 round magazine

Rifles

M4 Carbine

The M4 from the first game apears in Left 4 Dead 2. It can be seen in the demo video (located on IGN.com) showing the swamp campaign.

AKM

Said to be an AK-47 by Chet Faliszek in a recent interview at San Diego Comic Convention 2009, despite the fact that it looks exactly like a AKM. Since IMFDB judges video game guns by their looks, it's a AKM.

An AKM. What that else needs to be said?

FN SCAR-L

Seen in E3 gameplay video. Will likely replace the M16 from the first game as the 2nd-tier automatic. It fires from a 50 round magazine with is obviously make believe.

HK G3/SG1

A HK G3/SG1 can be seen laying the floor boards on top of some scaffolding in the last gameplay video. It is likely to replace the Hunting Rifle from the previous game. It seems to carry a 30 round magazine, double than the previous hunting rifle and 10 more than what the usual real life weapon can hold

Other

M79 Grenade Launcher

M79 grenade launcher - 40mm

This is seen in latest gameplay video and talked about in an interview with Chet Faliszek, project leader for Left 4 Dead 2.

Pipe Bomb

The pipe bomb from the first game is reintroduced. Like in the first game the pipe bomb has part of a smoke alarm attached to it that beeps. The beep sound atracts the zombie to a explosive demise. Unlike the first game zombie dont just exploded into a cloud of blood, but are tossed up and around from the explosion.

Molotov Cocktail

In one scene, a Molotov cocktail is shown. Likely re-introduced from the first game.